The present invention relates, in general, to downhole well detection, and more particularly to electromagnetic systems for downhole proximity detection while drilling.
In any drilling of boreholes in the earth, such as drilling oil and gas wells or drilling relief boreholes, it is essential for the driller to have as much information as possible about the location of the well being drilled with respect to surrounding strata and more particularly with respect to other wells which might be in the vicinity. For example, serious safety problems are present when drilling multiple wells from offshore platforms because of the need to avoid intersecting existing wells which are often very close to the well being drilled and where, during the first few thousand feet of drilling a new well, the planned distance from a neighboring well might be as little as six feet. Since it is not always possible to exactly control the location or direction of a drill bit, during this initial phase of drilling and during the initial turning of the well to direct it toward its targeted point in the earth, the danger of accidentally intersecting a nearby well is particularly great.
Similar safety problems exist when a relief well is being drilled to intersect with an existing well, for example, to bring a blowout well under control. In such a situation, it is extremely important to guide the relief well accurately to the existing, or target, well while closely monitoring the distance between the two and the direction of the target well from the relief well, so that the intersection takes place at the desired location in the shortest possible time. Thus, proximity detection and directional measurements are extremely important from a safety standpoint both when avoiding existing wells during the drilling of a new well, and when locating and intersecting a target existing well, as When drilling a relief well.
Numerous techniques have been developed for guiding boreholes, as exemplified by U.S. Pat. No. 4,072,200 to Morris et al, Pat. No. 4,458,767 to Hoehn, and by the several patents issued to the applicant herein, including Pat. Nos. 4,323,848, 4,372,398, 4,443,762, 4,529,939, and 4,700,142. In the patents issued to the applicant herein, an alternating current is injected from an electrode into the earth surrounding the well being drilled to produce a current flow in the surrounding strata. At least a part of that current flow concentrates on the casing of a nearby existing well, which current produces an alternating magnetic field which can be detected from the borehole being drilled. The detection of this magnetic field permits a determination of the direction and distance to the existing well. Although this system works very successfully, there is a need for a method of making such measurements while the relief well is being drilled. Such "measurement while drilling" or "MWD" apparatus would permit a continual evaluation of the proximity of nearby wells, without the need for stopping drilling, withdrawing the drill from the borehole, and inserting the measuring equipment. As a result, significant savings in time and cost would be effected. Such an MWD operation would be particularly valuable during the latter stages of drilling a relief well to intersect a blowout well, when the wells may be just a few meters apart from each other and continuous measurements may be critical.
An important factor in the drilling of wells is the ability immediately to communicate any data obtained to the surface. Numerous telemetry systems have been developed for this purpose, with one of the most widely available systems utilizing apparatus for generating pressure pulses in the well drilling fluid at the bottom of the well. These pulses are then detected at the surface.